1. Field of the Invention
The present invention relates to a tri-phase surge protector and its manufacturing method, and more particularly to a tri-phase surge protector and its manufacturing method that design a three-wire L-N-G protection on a zinc oxide ceramic body to improve the life and reliability.
2. Description of the Related Art
Referring to FIG. 1 for a bottom view and a side view of a prior art surge protector, the prior art surge protector includes a zinc oxide ceramic plate 6 having a conductive layer 7a on both top and bottom sides of the zinc oxide ceramic plates, a copper wire 8 (usually a tin plated copper wire) soldered on an electrode surface 7, an epoxy resin powder coated on an external layer for insulating electric charges and resisting moisture. The operating principle of the prior art surge protector uses a grain boundary phenomenon of the zinc oxide ceramic plate 6 to protect a voltage surge occurred at both ends and discharge surge energy by dissipating the heat, and thus its operation has to produce an electric current passing through the body of the zinc oxide ceramic plate 6 to convert surge energy into heat.
Referring to FIG. 2 for the schematic view of using a prior art surge protector to protect a three-wire L-N-G power supply, the surge protector is traditionally used for protecting a three-wire L-N-G power supply. Generally, three independent surge protectors 9 are used for protecting the power supply from being damaged by a surge occurred in the L-N-G terminals, wherein each surge protector 9 independently completes the protection between two wires.
Refer to FIGS. 3(a), 3(b), and 3(c) respectively for the bottom view, the side view, and the schematic equivalent circuit diagram of a three-terminal single-layer device as disclosed in JPN Pat. No. 59-5601, JPN Pat. No. 59-5601 based on the purpose of simplifying the circuit to design a three-terminal single-layer device 100. From its appearance, the three-terminal single-layer device 100 includes three terminals d, e, f, but the equivalent circuit diagram as shown in FIG. 3(c) shows that such method only protects the terminals d-f, and e-f, and there is no appropriate protection between the terminals d-e. Since there is no physical volume (grain boundary) between d and e for passing the terminal, therefore a breakdown will occur at d with respect to e under an appropriate electric field, and the surge energy cannot be removed by dissipating heat. There is no equivalent surge protector existed between d and e in the equivalent circuit diagram as shown in FIG. 3(c), and thus the design is incomplete for the protection.
Referring to FIGS. 4(a) and 4(b) respectively for the bottom view and the side view of a three-terminal single-layer device as disclosed in R.O.C. Pat. No. 313713, R.O.C. Pat. No. 313713 improved the design of the three-terminal single-layer device 120 as depicted in FIG. 3. From its appearance, the three-terminal single-layer device 120 includes three terminals g, h, k. In FIGS. 4(a) and 4(b), if an abnormal voltage occurs at each time of starting, it is necessary to have a breakdown (which produces a current) occurred at a terminal (assumed to be g in this embodiment) of the electrode 121 with respect to the electrode 123 on another side, and the electrode 123 produces a second breakdown (which produces a current) occurred at a terminal h of the electrode 122 or a terminal K of the electrode 123, and thus it is necessary to pass the current through the ceramic body twice for protecting the device each time. In other words, it is necessary to use the original designed area (121, 122 and 123) twice (121+122 or 122+123 or 121+123) to complete the protecting function. Such arrangement not only lowers the reliability, but also provides a resistance much smaller than three independent surge protectors (having a working area equal to 121 or 122 or 123) under the same lightning strike condition because the equivalent area is half of the original design.
Therefore, the present invention provides a tri-phase surge protector and its manufacturing method that design a three-wire L-N-G protection on a zinc oxide ceramic body to improve the product life and reliability.